Hmmm… In theory, your cooling system is continually vented via the radiator overflow pipe, so pressure shouldn’t build up, unless you have suffered such a seriously blown gasket that it was pumping engine compression into the system faster than the small bore overflow pipe could dump it. You should also still have compression on one cylinder, so it is all a bit odd. Let us know what is revealed when the engine is taken down. Your radiator sounds to be a post-war one, as a 1937 bike should have a honeycomb core radiator. Graham Moag can supply a new one if yours is beyond redemption. Good luck !
Brian
